Generally, in semiconductor devices using Metal-Oxide-Semiconductor (MOS) transistors, the Critical Dimension (CD) of a gate electrode greatly affects the characteristics of the MOS transistor. The higher the degree of integration of the semiconductor device, the lower the CD of the gate electrode and finally, it causes a short channel effect. To minimize the short channel effect, dopants for the adjustment of a threshold voltage are implanted. However, this method is also facing a technological limit due to the increase in the degree of integration. So the method of halo ion implantation, in which dopants are implanted only into the lower side of source and drain regions, has been proposed to improve such a short channel effect.
FIG. 1 is a cross-sectional view illustrating the semiconductor device in accordance with the prior method of Halo ion implantation. Specifically, a gate oxide layer 7 and a gate electrode 9 are formed on a silicon substrate 1 including the impurity regions of a low concentration (the region of N− 3) and a high concentration (the region of N+ 5). And nitride layers 11 and oxide spacers 3–13 are formed on both sidewalls of the gate electrode 9. In FIG. 1, number 15 indicates an oxide layer.
Accordingly, the prior method of the Halo ion implantation, wherein dopants are implanted using the gate electrode 9 and the oxide spacer 13 as a mask for the ion implantation, selectively forms an ion implantation region 17 only in the lower side of the source and drain regions including the impurity regions of the low concentration and the high concentration. Thus, it can prevent the rapid falling of the threshold voltage due to the short channel effect without the decrease of the channel mobility due to the whole increase of a threshold voltage.
However, for the prior method of halo ion implantation, because the regions for the halo ion implantation are formed in the area where the source and drain regions and the silicon substrate contact each other, a junction capacitance increases and a junction depth is affected. Therefore, it is difficult to adjust the threshold voltage of the device getting highly integrated.